1. Field of the Invention
This invention relates to a device and method for drilling a secondary borehole from an existing cased borehole in geologic formations.
More particularly, this invention comprises a three-in-one milling tool which has improved features when compared to prior art sidetrack casing milling operations.
2. Background
Previously drilled and cased wellbores, for one reason or another, may become non-productive. When a wellbore becomes unusable, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be sidetracked from the bottom of a serviceable portion of the cased borehole.
Sidetracking is often preferred because drilling, casing and cementing the borehole is avoided. Sidetracking involves milling through a steel pipe casing and should be accomplished without a major change in direction or dog leg in the borehole. This procedure is generally accomplished by either milling out an entire section of pipe casing followed by drilling through the side of the exposed borehole, or by drilling through the side of the casing with a mill bit that is guided by a wedge or "whipstock" component.
Drilling a sidetracked hole through a pipe casing is difficult and often results in unsuccessful penetration of the casing and destruction of the whipstock. In addition, if the window is improperly cut, a severely deviated dog leg may be the result rendering the sidetracking operation unusable.
Several patents relate to methods and apparatus to sidetrack through a cased borehole. U.S. Pat. No. 4,266,621 describes a diamond milling cutter for elongating a laterally directed opening window in a well pipe casing that is set in a borehole in an earthen formation. The mill bit has one or more eccentric lobes that engage the angled surface of a whipstock and cause the mill to revolve on a gyrating or non-fixed axis and effect oscillation of the cutter center laterally of the edge thus enhancing the pipe casing cutting action.
The foregoing system normally requires three trips in the sidetracking operation. A first stage begins a window in the well pipe casing, a second stage extends the window through use of a diamond milling cutter and a third stage with multiple mills elongates and extends the window.
While the window mill is aggressive in opening a window in the pipe casing, the number of trips required to complete the sidetracking operation (3) is expensive and time consuming.
U.S. Pat. No. 5,109,924 teaches a one trip window cutting operation to sidetrack a wellbore. A deflection wedge guide is positioned behind the pilot cutter and adjacent the end of the whipstock component. The pilot cutting tool or pipe casing mill is in such a position in the borehole that its frontal cutting surface does not come to rest on the ramped surface of the whipstock. In theory, the deflection wedge guide surface takes over the guidance of the cutting tool without the whipstock ramp surface being destroyed.
However, when a second and third milling tool of the same diameter and spaced one from the other a short distance behind the pilot mill, contacts the whipstock ramp, they mill away the guide. This inhibits or interferes with the pilot mill from sidetracking at a proper angle with respect to an axis of the cased borehole and may cause the pilot mill to contact the ramp surface of the whipstock before the cutter mill dears the pipe casing. The reamers or mills aligned behind the pilot mill having a diameter the same as (or larger than) the diameter of the pilot mill, prevents or inhibits the pilot mill from exiting the pipe casing easily. This is due to the lack of clearance space and flexibility of the drill pipe assembly making up the one trip window cutting tool when each of the following reamer mills sequentially contact the window in the casing. Hence, the sidetracking apparatus tends to mill straight.
U.S. Pat. No. 5,445,222 teaches a combination whipstock and staged sidetrack mill. A pilot mill spaced from and located on a common shaft above a tapered cutting end is, at its largest diameter, between 50 percent and 75 percent of the final sidetrack window diameter. A second stage cutting surface positioned on the same shaft and above the pilot mill being, at its smallest diameter, about the diameter of the maximum diameter of the pilot mill, and being, at its largest diameter, at least five percent greater in diameter than the largest diameter of the pilot mill. A final stage cutting surface, also on the same shaft, being at its largest diameter, about the final diameter, and at the smallest cutting surface diameter, being a diameter of at least about 5 percent smaller than the final diameter. The sidetracking mill is designed to accomplish the milling operation in one trip. The mill however, tends to go straight and penetrate the whip hence, the material of the whip must be harder than the casing to affect sidetrack. Otherwise, substantial damage to the whipface will occur and sidetracking may not occur as a result.
While the intent is to perform a sidetracking operation in one trip, difficulties often arise when attempting to deviate the drill string from its original path to an off line sidetracking path. Progressively larger in diameter reaming stages to enlarge the window inhibits the drill shaft from deviating or flexing sufficiently to direct the drill pipe in a proper direction resulting in damage to the whipstock and misdirected sidetracked boreholes. In other words, the sidetracking assembly tends to go straight rather than deviate through the steel casing.